The present invention relates to creation and viewing of three-dimensional moving or still images, and in particular to xe2x80x9cautostereoscopicxe2x80x9d systems that do not require special wearable peripheral devices.
For well over a century, researchers have proposed and developed a variety of devices that allow viewing of images or graphic designs in three dimensions. The concept of true xe2x80x9cstereoscopicxe2x80x9d viewing holds tremendous potential for applications ranging from entertainment to scientific visualization and basic research. Stereoscopically portrayed stories and events carry the impact and immediacy of genuine realism, allowing viewers to be drawn into an experience with the sense that they are truly there.
Efforts to date have largely been confined to specific, controlled environments. For example, for decades movie-goers have donned special glasses to view stereoscopic films; for this application, the restricted presentation atmosphere and a captive audience willing to tolerate special viewing aids facilitated use of relatively unsophisticated optical arrangements. Similar audience receptivity underlies the current generation of commercial xe2x80x9cvirtual realityxe2x80x9d devices, which require the user to wear full-vision headgear that imparts an xe2x80x9cimmersivexe2x80x9d three-dimensional environment.
Displays involving eyeglasses or headgear control what enters the eyes rather than what exits the display. The wearable apparatus covering the user""s eyes allows separate information to be provided to each eye. The left-eye and right-eye images differ in perspective but not content, so the viewer integrates the images into a single, stereoscopic picture. Early three-dimensional film systems displayed left-eye and right-eye images in separate colors, which were directed to the appropriate eye by special glasses having lenses tinted with one or the other of these colors. More recent adaptations of this approach code the left-eye and right-eye images with orthogonal polarizations, and utilize eyeglasses with orthogonally oriented polarizers.
The disadvantages to systems that require eyeglasses or wearable headgear are numerous and well-known. Beyond the inconvenience and unnaturalness of wearing an appliance, the user may also experience headaches or eye strain. Head-mounted displays suffer the additional disadvantage of being single-user devices, isolating viewers from one another and preventing them from sharing the threedimensional experience with others.
Another popular form of stereoscopic display relies on lenticular optical technology, which utilizes a linear array of narrow cylindrical lenses to create separate spatial viewing zones for each eye. Image information for the different viewing zones is spatially separated in the back focal plane of the cylindrical lenslets, allowing the lenslets to direct this information only to a narrow area of the viewing plane. Recent adaptations of this approach utilize liquid crystal display (LCD) panels or LCD-projected images to provide an updatable display medium for creating the spatially separated information behind the cylindrical lenses. Lenticular displays also suffer from certain drawbacks, however, such as poor image resolution (due both to the need to divide the overall resolution of the single image-producing device over a plurality of viewing zones, and to diffraction). Lenticular designs are also difficult to adapt to multi-user environments.
Other approaches to stereoscopic image presentation include so-called xe2x80x9cvolumetricxe2x80x9d displays (which utilize a medium to fill or scan through a three-dimensional space, small volumes of which are individually addressed and illuminated), and electronic holography displays. Both of these types of display require rapid processing of enormous quantities of data, even for lower resolution images, and both have significant obstacles to overcome when the displays are scaled up to accommodate larger image sizes. In addition, the volumetric displays produce transparent images which, while suitable for applications (such as air-traffic control or scientific visualization) where the illusion of solidity is less important than a wide viewing zone, do not typically provide a fully convincing experience of three-dimensionality.
Macro-optic display systems utilize large-scale optics and mirrors, as opposed to lenticular displays, to deliver each image of a stereopair to a different viewing zone. A system designed by Hattori et al. (see Hattori et al., xe2x80x9cStereoscopic Liquid Crystal Display,xe2x80x9d Proc. Telecom. Advance. Org. (TAO) 1st Int""l. Symp. (1993)) utilizes two LCDs, one providing left-eye information and the other providing right-eye information. The outputs of both LCDs are combined by a beamsplitter, with the light passing through each LCD being focused to a separate viewing zone. The Hattori et al. system utilizes a monochrome cathode-ray tube (CRT) monitor behind each LCD as the illuminating light source. Each monochrome monitor is driven by a camera that records the viewing area in front of the display, capturing a picture of the viewer. A pair of infrared (IR) illuminators, each emitting at a different wavelength, are angled toward the viewer from different sides. Each recording camera is equipped with a bandpass filter tuned to the emitting frequency of one or the other IR illuminator. Because the illuminators each face the viewer at an angle, one of the cameras records an image of the left side of the viewer""s face, while the other records an image of the right side. A Fresnel lens near each LCD projects the left-side or right-side image of the viewer""s face, as appropriate, onto the corresponding side of the viewer""s actual face. As a result, the image information from each LCD reaches only the appropriate left or right eye; for example, because light passing through the left-eye image LCD goes only to the left side of the viewer""s face, the viewer""s left eye sees only left-eye information. As the viewer moves within the viewing space, the image on the monitors moves with him, so the viewing zones for the left-eye and right-eye images remain properly positioned over the viewer""s left and right-eyes.
This type of display offers a number of advantages over prior designs. It is xe2x80x9cautostereoscopic,xe2x80x9d so that the user receives a three-dimensional image without special wearable peripheral devices. It is capable of delivering three-dimensional images to a moving viewer anywhere within the viewing area, and can accommodate several viewers at once. In addition, because the system uses LCDs as the primary image source for the stereopairs, it is capable of generating strongly realistic, full-color images of naturalistic scenes (as well as displaying ordinary two-dimensional television or other information). And since only two LCDs are used, the total amount of information needed to drive the display is only twice that of a standard television or monitor.
The Hattori et al. design poses problems in terms of scalability, however. Because the viewer-tracking system is implemented with CRTs, larger displays will require proportionally larger CRTs and more powerful lenses. As a result, the display size, cost, and complexity expand dramatically with increase in the size of the stereoscopic image. Moreover, because this design requires a final beamsplitter to be placed between the display medium and the viewer, the resulting three-dimensional images give the psychological impression of being inaccessible to the viewer; this arises from the fact that stereoscopic images display the least distortion when the three-dimensional content is localized at or near the surface of the display medium, which is positioned behind the final beamsplitter. Other limitations of the design stem from shortcomings that generally affect CRT displays, such as varying image intensities.
Another macro-optical design was recently proposed by Ezra et al. (see Ezra et al., xe2x80x9cNew Autostereoscopic Display System,xe2x80x9d SPIE Pro. #2409 (1995)). Starting once again with two image LCDs combined by a beamsplitter, an additional system of fold mirrors and another beamsplitter were added to allow both LCDs to be backlit by a single light source. A lens disposed near each LCD images the single light source to the eyes of the viewer. These lenses are laterally offset by a slight amount from the optical axis of the LCDs so that two separate images of the light source are created in the viewing area. Light passing through the left-eye LCD forms an image of the light source near the viewer""s left eye, while light passing through the right-eye LCD forms an image of the light source near the viewer""s right eye. By moving the light source behind the two LCDs, the left-eye and right-eye viewing zones can be moved to follow a viewer within the viewing area. To accommodate additional viewers, additional xe2x80x9cdynamic light sourcesxe2x80x9d can be added so as to create further viewing zones. More recently, this group proposed handling multiple viewers with a single, specialized illumination component rather than multiple individual light sources. This specialized component consists of a number of thin, vertical cold-cathode sources arranged in a one-dimensional array. See Woodgate et al., xe2x80x9cObserver Tracking Autostereoscopic 3D Display Systems,xe2x80x9d SPIE Proc. #3012A (1997).
This system shares many of the advantages of the Hattori et al. design described above, and overcomes the difficulties stemming from multiple illumination sources. Once again, however, the ability to scale the Ezra et al. system can be problematic. The two LCDs and the beamsplitter occupy a large space for a display of only modest three-dimensional size. As in the Hattori et al. system, the three-dimensional image is xe2x80x9ctrappedxe2x80x9d behind the output beamsplitter, making the images seem inaccessible. Finally, the array of cold-cathode sources have a limited on/off switching speed, creating possible lags in tracking speed.
The present invention offers an autostereoscopic display system that is capable of tracking one or more users within a viewing zone and presenting complementary stereoimages to each of the viewer""s (or viewers"") eyes. More broadly, the invention is useful in any context requiring the directing of separate images to distinct spatial regions. For example, the images generated by the invention can be directed to different viewers (rather than the left and right eyes of a single viewer), so that viewers observing the same display device can view different images. Although the ensuing discussion is directed toward generation of stereoimages for a single viewer, it should be understood that this is for convenience of presentation only.
In accordance with the invention, polarization is used to segregate the images that are to be directed to different spatial regions. The two images are interdigitated within a single displayxe2x80x94that is, each row (or column) of image points from the first image alternates with a row (or column) of image points from the second image on the display. (For ease of explanation, each row or column of image points is herein referred to as a xe2x80x9cbandxe2x80x9d so as not to specify a vertical, horizontal or other orientation.) As a result, the resolution of the image is half the resolution of which the display is capable, since that resolution is split between the two images. The bands of the first image are polarized in a first direction and the bands of the second image are polarized in a second direction orthogonal to the first. This may be accomplished, for example, using an LCD in which alternating, contiguous bands of pixels correspond to bands of one or the other image, and a xe2x80x9cpatterned polarizerxe2x80x9d also organized into bands that are aligned with the pixel bands.
Such differentially polarized images could be viewed, for example, through conventional polarized xe2x80x9c3D eyeglassesxe2x80x9d in which each eyepiece is a polarizer oriented in a different direction, i.e., the first or second direction. Since these directions are orthogonal, each of the wearer""s eyes would see only one of the two images. But the present invention facilitates autostereoscopic viewing by a user who is stationary or moving within a viewing zone. This is accomplished by utilizing a display (such as an LCD) that is illuminated by light from an outside source whose polarization can be controlled. Illumination from the light source passes through a focusing lens and then through the patterned polarizer and display so as to form a xe2x80x9crealxe2x80x9d or xe2x80x9caerialxe2x80x9d image of the display in the viewing zone.
For stereoscopic applications, a viewer-locating means may acquire a facial image fragment of the viewer; as used herein, the term xe2x80x9cfacial image fragmentxe2x80x9d refers to a portion of the viewer""s face or a bounded region that includes a portion of the viewer""s face, and may include, or be limited to, a first one of the viewer""s eyes. For example, a camera at the front of the display may capture a picture of the viewer with one side of his face illuminated by an IR source. This embodiment further comprises means for generating a tracking output image, the output image comprising a first region of light polarized in the first polarization direction and substantially conforming to the facial image fragment, and a second region of light polarized in the second polarization direction (orthogonal to the first polarization direction). The facial image fragment is focused onto a corresponding portion of the viewer""s face through the display. As a result, this arrangement presents illumination from the first-image bands to the first eye of the viewer and illumination from the second-image bands to the viewer""s other eye. This is because the user""s first eye receives light polarized in the first direction (and therefore sees only the first image) while the user""s other eye receives light polarized in the second direction (and therefore sees only the second image). The images may represent a single, still stereopair or may instead change rapidly over time to convey movement.
Alternatively, differential polarization of the illumination source can be used to provide different images to different viewers within the viewing zone. Indeed, using a robust tracking system capable of locating not only multiple viewers but the positions of their eyes as well, it is possible to provide the illumination source with a pattern of different polarizations that direct the different stereoimages to the proper eyes of all viewers.
The invention also embodies methods relating to the above-described systems and various components thereof.